It is known practice to separate such a mixture by cryogenic distillation, using at least one stripping column which serves to reduce the H2 content of the CO, followed by a nitrogen-removal column which is a CO/N2 separation column.
Head condensers at the top of the CO/N2 columns are of the “water bath vaporizer” type.
When the N2 content in the mixture (which means to say the N2 content entering the CO/N2 column) is far lower than the content used as a basis for designing the CO/N2 column, the installed condenser has, de facto an excess heat-exchange surface area which leads to all of the CO/N2 mixture becoming condensed at the bottom of the tank, the result of this being that the column operating pressure can no longer be maintained. It is then no longer possible to maintain the purity of the CO that is to be produced, and that results in a risk of the unit shutting down. One way of reducing the exchange surface area of the condenser is to lower the level of the bath (liquid CO—partially “unflood” the condenser) but there is a limit to this principle caused by the fact that the condenser is of the “thermosiphon” type, namely is designed for a recirculation rate that requires a minimum liquid charge in order to operate correctly, the minimum head of liquid required not necessarily being aligned with the head to which it needs to be set in order to manage very low N2 contents.
The risks associated with excessively low N2 contents in the CO/N2 column are generally compensated for through the possibility of importing and injecting gaseous N2 into the impure-CO line feeding the column in order to guarantee the operational stability of the “water bath vaporizer” condenser and the stability of the operating pressure in the column.
FR-A-2895067 and WO-A-2008/099124 describe solutions using an addition of nitrogen if the nitrogen content in the mixture drops.
EP-A-0928936 and “Cryogenic Gas Separation: The most economic and experienced separation process for production of carbon monoxide and hydrogen from raw synthesis gas” by Linde AG describe methods according to the closest prior art.